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Abstract-Carbon-13 N M R spectra of a series of p-diketones in the enol form with various /I-substituents have been studied. An additive influence of the //-substituents on the chemical shifts of the carbon atoms in the hydrogen bonded chelate ring has been found. It is shown that the c[-and P-carbon chemical shifts can be calculated by means of a set of increments for arbitrary combinations of the X and Y substituents. Analysis of the experimental data enables the conclusions to be drawn that enol-enolic tautomerism with different populations of the forms (A) and (B) is absent in /i-diketones and that the carbon chemical shift changes are caused by electron density redistribution in the hydrogen bonded chelate ring (C).

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